The present invention relates to a master cylinder used in a brake system of a motor vehicle or the like, a method of manufacturing the master cylinder, and a check valve system used in the master cylinder.
In the master cylinder, a piston is slidably fitted in a cylinder body to which operating fluid is introduced from a reservoir, the cylinder body defining therein a pressure chamber in which pressure is applied to the operating fluid according to an operation of the piston. The pressure chamber is connected via a pipe to a hydraulic instrument of a brake system or the like, such that the hydraulic instrument is operated according to an operation of the piston. The cylinder body is provided with a supply passage for supplying the operating fluid from the reservoir to the pressure chamber, so as to prevent a pressure in the pressure chamber from becoming negative during an operation of returning the piston or the like.
In recent years, development has been in progress for traction control systems for automatically applying brake forces to wheels according to vehicle wheel slip conditions. In such a system, a control pump, which is a part of a hydraulic instrument, sucks operating fluid from a pressure chamber of a master cylinder and supplies the operating fluid to a wheel brake unit during a traction control. During this operation, in the master cylinder, an amount of operating fluid corresponding to the amount of operating fluid sucked from the pressure chamber is supplied from the reservoir via the supply passage to the pressure chamber. However, the supply of operating fluid is likely to be insufficient when the control pump sucks a large amount of operating fluid.
Conventionally, known master cylinders directed to this problem have a structure adapted to open a separate passage connecting the reservoir and the pressure chamber when the pressure chamber does not contain enough operating fluid (refer, for example, to Japanese Patent Public Disclosure No. HEI 11-268629). Such a master cylinder is provided with a bypass in a cylinder body thereof, the bypass bypassing a supply passage and connecting a reservoir and a pressure chamber. The bypass is provided therein with a check valve that is adapted to open when the pressure in the pressure chamber becomes lower than that in the reservoir. The check valve comprises: a valve seat member having a valve hole; a valve body adapted to detachably make contact with a valve seat of the valve seat member; and an urging spring for urging the valve body toward the valve seat. These members are disposed in a valve chamber formed in the bypass.